I'm just starting to use okps and last Saturday had a 'nearly but not quite as dark' test line. The next day the line was barely visible and then the next day faded completely. I had no other signes that I'd O'd, so I took it as a negative and further tests were also clearly negative. So I began to wonder if I had O'd. Tested at about 12 noon yesterday with Fortel and completely negative - no second line - but then tested at about 4pm in the afternoon with Confirm and got a very clear second line - although still not stronger than the C line. I'm going to continue with Confirm and see how that works for me.
I've recently come off the pill and have no idea what my cycle is (so far have not had a regular on) so for me so it's very much still the learning phase.
Good luck